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Camera inside walls and use of stencil buffer

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What I really want to do, is to draw something proper instead of seeing through walls. Given a projection matrix based on a view frustum, is it possible to have bits set in a stencil buffer when a write fails the near plane cut-off?

There is a possibility to control stencil bits from failed depth tests, but I don't know if culling because of being outside the cut-off planes counts.

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What you mean is "clipping", and yes, it happens before any fragments are produced, so no what you describe is not possible. Can you clarify what it is you want to do?

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What you mean is "clipping", and yes, it happens before any fragments are produced, so no what you describe is not possible. Can you clarify what it is you want to do?

I want to detect fragments that are clipped by the near plane, to make it possible to draw something else. But as you say that clipping is done before fragments are produced, the idea doesn't seem to be possible. Maybe there is another way to get it done?

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To me at least, it is still not very clear what you are trying to achieve.
As for your direct question, you could have a look at the depth of the fragment you are drawing in your pixel shader, using eg. gl_FragCoord.z. You can compare this value to 0, which will usually be the depth value at the near-plane, possibly shade the fragment in a different way, and then modify the depth that is used to compare it using gl_FragDepth.

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